1. Field of the Invention
The present invention relates generally to methods and apparatus for detonating downhole explosives such as well perforating guns proximate of well terminus structure.
2. Description of Related Art
After a well is drilled, the open well bore is often cased to maintain the integrity of the production face. Some well completion procedures may include bottom hole plug structure to seal the well bore below a fluid mineral bearing production zone. That portion of the casing length adjacent to the production zone is perforated to admit the flow of in situ formation fluids into the casing bore. In other well completion procedures, the in situ fluid may be produced from an uncased production face. In either case, the in situ formation fluid is delivered to the well surface along the bore of a production tube that is suspended from the surface along the axial length of the well.
The production tube often penetrates a packer structure which seals the annulus between the outer perimeter of the production tube and the interior of the casing or raw bore wall above the formation fluid production zone. Below the formation production zone, the production tube may be plugged.
Like the casing, the production tube wall is perforated along that length section proximate of the formation production face to admit entry of formation fluid into the tube flow bore.
Pipe and casing perforations such as described above are often produced by a multiplicity of shaped charge explosives distributed along the length and around the perimeter of a cylindrical perforating gun. Shaped charges are usually fabricated of high order explosive that are, in some circumstances, difficult to detonate. Historically, numerous techniques have been used to detonate such shaped charges. For example, perforating guns have been actuated electrically, by means of a drop bar mechanism and by fluid pressure upon a firing head. Many complex factors contribute to a decision regarding which of these firing mechanisms is most appropriate for a specific well completion. The present invention addresses a method and apparatus for activating the firing head by fluid pressure.
When a firing head is activated by annulus fluid pressure, the entire gun length and the complete well environment is also subjected to an activation pressure that is over and, above the in situ well pressure. In a very deep well, the summation of both in situ pressure and activation pressure may be so great as to inhibit the shaped charge detonations and perforation depth. Increased well pressure may also crush the perforating gun or create an unwanted breach in the well casing or tubing.